1. Field of the Invention
This invention relates to floating vessels useful for offshore oil and gas drilling and production operations in general, and in particular, to a deep-draft, semi-submersible, extendable-draft platform (“EDP”) that incorporates a single, centrally located support column to minimize platform cost, complexity and motions resulting from surface wind and wave forces.
2. Related Art
The development of deep water offshore oil and gas fields, such as are found in the Gulf of Mexico and the North Sea, present substantial challenges to the petroleum industry. Early production schedule requirements necessitate maximal inshore integration and commissioning, together with a year-round deployment capability. Moreover, the ability to use so-called “dry trees” and steel catenary risers (“SCRs”) requires that the motions of the deployed structures in response to wind and waves be relatively small, even in seasons with rough seas.
Such offshore oil and gas operations involve the provision of a vessel or working platform in which the drilling, production and storage equipment, together with the living quarters of the personnel manning the platform, if any, are integrated. In general, offshore platforms fall into one of two groups, viz., “fixed” and “floating” platforms. Fixed platforms comprise a “topside,” or equipment deck, that is supported above the water by legs that extend down to and are seated, directly or indirectly, on the sea floor. While relatively stable, such fixed platforms are typically limited to shallow waters, i.e., depths of about 500 feet (150 m) or less.
Floating platforms are typically employed in water depths of 500 ft. (154 m) and greater, and are held in position over the well site by mooring lines or chains anchored to the sea floor, or by motorized thrusters located on the sides of the platform, or by both. Although floating platforms are more complex to operate because of their greater movement in response to wind and wave conditions, they are capable of operating at substantially greater depths than fixed platforms, and are also more mobile, and hence, easier to move to other offshore well sites. There are several known types of floating platforms, including so-called “drill ships,” “tension-leg” platforms (“TLPs”), “spar” platforms (“SPARs”), and “semi-submersible” platforms.
Spar platforms comprise a single, long, slender, buoyant hull that gives the platform the appearance of a column or spar when floating in an upright operating position, in which an upper portion of the platform extends above the waterline and a lower portion is submerged below it. Because of their relatively slender, elongated shape, they provide structural simplicity and compactness, and present a smaller area of resistance to wind and wave forces than do other types of floating platforms. However, the offshore installation of their equipment deck requires the use of a heavy-duty crane barge. Additionally, they have a relatively low hull efficiency, and their off-shore hook-up and connection procedures are relatively time-consuming and expensive. Examples of spar-type floating platforms useful for oil and gas exploration, drilling, production, storage, and gas flaring operations may be found in, e.g., U.S. Pat. Nos. 6,213,045; 5,443,330; 5,197,826; 4,740,109; 4,702,321; 4,630,968; 4,234,270; 3,510,892; and 3,360,810.
Conventional semi-submersible offshore platforms are used primarily in offshore locations where the water depth exceeds about 300 ft. (91 m). This type of platform comprises a hull structure that has sufficient buoyancy to support the equipment deck above the surface of the water. The hull typically comprises one or more submersible “pontoons” that support a plurality of vertically upstanding columns, which in turn support the deck above the surface of the water. The size of the pontoons and the number of columns are governed by the size and weight of the deck and equipment being supported.
A typical shallow-draft semi-submersible platform has a relatively small draft, typically, about 100 ft. (30.5 m), and incorporates a conventional catenary chain-link spread-mooring arrangement for station-keeping over the well sites. The motions of these types of semi-submersible platforms are relatively large, and accordingly, they require the use of “catenary” risers (either flexible or semi-rigid), extending from the seafloor to the work platform because they are not capable of supporting the other types of risers, e.g., top-tensioned vertical risers (“TTRs”) that are typically employed in deep water operations. Also, heavy wellhead equipment must typically be installed on the sea floor, rather than on the work platform, for the same reason. Typical semi-submersible offshore platforms are described in the following references: CA 1092601; GB 2,310,634; U.S. Pat. No. 4,498,412; WO 85/03050; GB 1,527,759; WO 84/01554; GB 2,328,408; U.S. Pat. No. 6,190,089; GB 1,527,759; and, WO 02/00496.
An “extendable draft” platform, or “EDP,” comprises a buoyant equipment deck having a plurality of openings (“leg wells”) through the deck. Depending on the application, the deck may be rectangular or triangular, with a leg well at each corner or apex, although other configurations may also be used. A buoyancy column that can be ballasted (e.g., with seawater) is installed in each of the leg wells. The columns are initially deployed in a raised position, and then lowered to a submerged position after the EDP has been moved to a deep water site. Each column is divided by internal bulkheads into a plurality of compartments, and includes means for controllably forcing seawater ballast into and out of selected ones of the compartments to adjust the vertical position of the columns in the water, and hence, the draft of the platform. A “heave plate” or pontoon assembly is attached to the bottom of the columns to help stabilize the EDP against the heave action of wind, waves and swells.
One of the advantages of EDPs is that they are “self-installing,” i.e., the hull and topside can be integrated at ground level at the fabrication yard, and no barge crane is required for off-shore deployment. However, they are still subject to increased current motions when compared to, e.g., SPAR platforms, because they typically incorporate from three to nine upright support columns, which extend up through the surface of the water and thereby increase the effective area of resistance of the platform to wind, wave and current forces acting at that level, as compared to a SPAR platform, which exposes only a single, long, slender, buoyant hull to such forces. Additionally, the use of multiple columns entails higher fabrication costs, and the leg wells are relatively wasteful of useful equipment deck area.
It would therefore be desirable if the structural simplicity, compactness and small exposure to wind, wave and current load of a SPAR platform could be combined with the inshore hull and topside integration and self-installing features of a deep draft EDP.